/Engine/Source/Runtime/SlateCore/Public/Brushes/SlateBoxBrush.h
A 3x3 box where the sides stretch horizontally and vertically and the middle stretches to fill. The corners will not be stretched. The size of the sides and corners is determined by the Margin as follows:_________
struct FSlateBoxBrush : public [FSlateBrush](API\Runtime\SlateCore\Styling\FSlateBrush)